commandline --showing-suggestion to ignore single-space autosuggestion

All-whitespace autocompletions are invisible, no matter the cursor
shape.  We do offer such autosuggestions after typing a command name
such as "fish". Since the autosuggestion is invisible it's probably
not useful. It also does no harm except when using a binding like

	bind ctrl-g '
	    if commandline --showing-suggestion
	        commandline -f accept-autosuggestion
	    else
	        up-or-search
	    end'

where typing "fish<ctrl-g>" surprisingly does not perform a history
search.  Fix this by detecting this specific case. In future we
could probably stop showing autosuggestions whenever they only
contain whitespace.

src/reader.rs

@@ -963,7 +963,10 @@ pub fn reader_showing_suggestion(parser: &Parser) -> bool {
     }
     if let Some(data) = current_data() {
         let reader = Reader { parser, data };
-        !reader.autosuggestion.is_empty()
+        let suggestion = &reader.autosuggestion.text;
+        let is_single_space = suggestion.ends_with(L!(" "))
+            && reader.command_line.text() == suggestion[..suggestion.len() - 1];
+        !suggestion.is_empty() && !is_single_space
     } else {
         false
     }